An organic thin film solar cell is a device using a donor material and an acceptor material together for a photoactive layer, and has advantages in that as compared to conventional inorganic semiconductor devices, a film forming condition is not complicated, and a material of a photoactive layer having a thin thickness of several hundreds of nanometers or less and being relatively inexpensive, in particular, a flexible device that is bendable freely, is capable of being produced. Thus, a number of researches on the organic thin film solar cell are underway.
The organic thin film solar cell has a bonding structure of an electron donor and an electron acceptor, and exhibits a very rapid charge transfer phenomenon which is so-called “photoinduced charge transfer (PICT)” between the electron donor and the electron acceptor, i.e., a photovoltaic effect.
When an organic semiconductor compound used as the electron donor is a semiconductor polymer, various derivatives of a poly para-phenylenevinylene (hereinafter referred to as “PPV”)-based material and polythiophene (hereinafter referred to as “PT”) are used. Further, C60 fullerene itself or a C60 fullerene derivative designed to easily dissolve C60 fullerene in an organic solvent is used as the electron acceptor, and perylene, 3,4,9,10-perylene tetra carboxylic acid diimide, phthalocyanine, pentacene, etc., are used as other small molecules.
In order to increase efficiency of the solar cell, a contact area between the electron donor and the electron acceptor should be large, and two separated charges should be movable to an electrode without loss of charge.
Thus, researches on various electron donors are underway. For example, U.S. Patent Application Publication No. 2006-0011233 describes an organic photovoltaic device in which poly (3-hexylthiophene) (P3HT) is used as an electron donor, [6,6]-phenyl-C61-butyric acid methyl ester (C60-PCMB) is used as an electron acceptor, and a photoelectric conversion layer is introduced by a spin coating method.
Up to now, a research on an electron donor has been numerously conducted, but a research on a compound for replacing a fullerene derivative used as the electron acceptor has been hardly conducted.
Since a large number of fullerene derivatives have low solubility with respect to an organic solvent, a phase separation phenomenon occurs when mixed with a polymer, and thus an efficiency in view of an appearance is generally low, light absorption with respect to a range of a region at which solar spectrum is strong is weak, an operation for energy level is difficult, and miscibility with the electron donor is low. Thus, a research on a compound for replacing fullerene is urgently needed.
Specifically, as the compound for replacing the fullerene derivative, a compound having high electron affinity similar to that of fullerene while simultaneously having excellent miscibility with the electron donor, high absorption coefficient, and excellent photo conversion efficiency, is required to be studied.